According to Official Guinness Records,
The largest tank battle ever is generally acknowledged to be the Battle of Kursk during World War II. On 12 July 1943, in the Prokhorovka region near the Russian town of Kursk, a total of 1,500 German and Russian tanks amassed for close-range fighting. By the end of the day both sides had lost over 300 tanks each. The battle was part of a larger German offensive in Russia, known as Operation Zitadelle (Citadel) from 5 July - 22 August 1943. Although it was supposed to be a surprise offensive by the Germans, the Russian Red Army was secretly informed of the plan. By the end of the campaign, the Russians had successfully ended the German campaign in their homeland, but casualites were high: 500,000 German soldiers were killed/wounded, as for the Russians - 250,000 killed, 600,000 wounded and 50% of their tank strength was lost. The total amount of armour amassed in preparation for Operation Zitadelle was: 900,000 German soldiers, 2,700 tanks, 2,200 aircraft and on the Russian side 1,300,000 men, 3,600 tanks, 20,000 artillery pieces and 2,400 aircraft.
The battle marked the decisive end of the German offensive capability on the Eastern Front and cleared the way for the great Soviet offensives of 1944–45.
For a complete list of 1943 records, please visit 1943 Guinness Records in Russia.